Liquid crystal displays are formed of a nonisotropic liquid which is birefringent and exhibits interference patterns in polarized light; this behavior results from the orientation of molecules parallel to each other in large clusters. Liquid crystal displays (LCDs) are constructed as a common back plane electrode and a series of segment electrodes formed of transparent conductors. Depending upon the magnitude of voltage applied between the back plane conductor and individual segment conductors, characters are formed by light reflected from the particular segments upon which the display voltage is applied. Because LCD devices have a very high internal impedance, the current drawn by the devices is very small making LCDs desirable for battery operated instrumentation. There is, however, a tendency for the transparent conductors to become obscured as a result of molecular migration caused during application of a sustained DC voltage. LCD devices are thus typically operated using an AC applied voltage rather than a sustained DC voltage.
In prior art non-multiplexed LCD drive systems, the AC drive signal is generated using parallel inversion provided by multiple Exclusive Or gates or data selectors. This, however, is a very hardware intensive system and becomes costly as the number of driven lines increases. A recently developed solution to this parallel drive problem involves LCD multiplexing to substantially reduce the number of lines to be driven. There has, however, been a concomitant increase in the complexity of driving those lines. For example, multiple voltages must be used at the output of the drive, and these voltages must be controlled to change with temperature in proportion to the LCD threshhold voltage. Furthermore, multiplexing requires the LCDs to respond much more quickly to changing input voltage, and to have a substantially smaller and better defined region of on-off uncertainty than in the parallel drive approach. These factors increase the cost of the drivers and LCD, making it cost effective in only large systems.
One object of the present invention, therefore, is to provide an LCD drive circuit having reduced complexity compared to that of conventional, multiplexed LCD drive circuits.
Another object is to provide an LCD drive circuit using a minimum number of components independent of the number of driven lines in the system.
Another object is to provide an LCD drive system that prevents damage to the LCD that would tend to occur by prolonged application of DC voltage.
Yet another object of the invention is to provide a low cost LCD drive circuit that is suitable for instrumentation applications.